John McCain's hold on the Republican nomination tightened yesterday when he received the unexpected endorsement of his previously bitter rival, Mitt Romney.

With the party closing ranks ready for November's presidential election, Romney put aside his political and personal differences to back McCain.

Romney, who dropped out of the race last Thursday, proposed that the 288 delegates he had accumulated should change their support to McCain. Romney released them from their pledges.

Not all of them will go to McCain, but, if enough do, he will be close to the 1,191 he needs to wrap up the contest. McCain has 843 at present.

In a joint appearance in Boston, Romney played down the personal dislike between them: "Even when the contest was close and our disagreements were debated, the calibre of the man was apparent."

Romney said he had never lost sight of McCain's qualities as a leader in time of war and, unlike the Democratic candidates, he was a real American hero.

During the contest McCain complained about Romney's underhand tactics, but yesterday he described him as having fought "an honourable" campaign. McCain added that primaries were often tough, and he had emerged from the battle with Romney as a better candidate.

Romney's endorsement opens the way for him to join McCain later in the year as the vice-presidential candidate. The latter said he expected Romney to campaign alongside him later this year.

The pair cut the press conference short to avoid answering questions about a vice-presidential running mate.

Romney's conservative background would help McCain, who is more moderate, with the Republican party base.

But the personal animosity that exists between them would make it awkward for them to run together. Romney may instead present himself as a party loyalist, thereby preparing the ground for another bid by him for the nomination in four years' time, should McCain fail to win the White House.

The two had not spoken since Romney pulled out last week. The endorsement was negotiated by advisers. It adds to pressure on Mike Huckabee, still in the race but trailing badly, to drop out. Ron Paul is also still in the race, but as a marginal figure. Although Huckabee insists he will continue, McCain is uncatchable, even before Romney's announcement.

Meanwhile, Hillary Clinton finally won the popular vote in New Mexico's Democratic caucus and picked up one extra delegate last night, nine days after "Super Tuesday" voting ended.

A hand count of 17,000 provisional ballots given to voters on February 5 because of long lines and a shortage of ballot papers gave Clinton a 1,709 vote edge over Barack Obama, with 73,105 to 71,396. Clinton has 14 delegates in the state to 12 for Obama. The national delegate count stands at 1,276 for Obama and 1,220 for Clinton.